This disclosure relates to seismic exploration for oil and gas and relates, in particular but not by way of limitation, to seismic data acquisition with time-distributed sources and the processing of the acquired data.
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey may involve deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological formations, creating pressure changes and vibrations along the way. Changes in elastic properties of the geological formation scatter the seismic waves, changing their direction of propagation and other properties. Part of the energy emitted by the sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure changes (hydrophones); others are sensitive to particle motion (e.g., geophones). Industrial surveys may deploy one type of sensor or both types. In response to the detected seismic events, the sensors generate electrical signals to produce seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.
Some surveys are known as “marine” surveys because they are conducted in marine environments. However, “marine” surveys may be conducted not only in saltwater environments, but also in fresh and brackish waters. In one type of marine survey, called a “towed-array” survey, an array of seismic sensor-containing streamers and sources is towed behind a survey vessel. Seismic surveys may be conducted in an area between land and sea, which is referred to as the “transition zone”. Other surveys, incorporating both hydrophones and geophones, may be conducted on the seabed.
In marine surveys, airguns or arrays of airguns are popular seismic sources. To generate impulsive far-field signature, similar to those generated by dynamite, airguns in an airgun array of different size or character are arranged in certain geometric arrangements and are activated according to certain time sequence such that the generated wave fields are overlapped constructively or destructively to form impulsive source signature at the far field. The energy of the waves can be concentrated in a time and space during the wave propagation into the Earth. This can be environmentally damaging to the marine life in the surveying area and other sensitive marine structures. It is desirable to reduce the peak energy to reduce the environmental impact during seismic survey.
Instead of optimizing the airguns in an airgun array to form an impulsive source signature, (i.e. tuning the airgun array), there is a method called “popcorn” or “machine gun” firing. In this method, an individual airgun in an airgun array is fired at random (or pseudo random) times. Thus, the energy from the airgun array is distributed across a pre-defined time interval. The peak energy from the airgun array is much reduced.
There are a number of benefits to this type of acquisition, including reducing the peak output of an airgun array and reducing cross-talk between simultaneous seismic sources. However, whereas the tuned airgun array is designed to have the desired broadband spectral output, the distribution of the array in time is a significant de-tuning operation. The resulting output may be broadband in the sense that it spans the same frequency range, but by spreading the individual airgun signatures across time, a number of notches are introduced into the source spectrum. These notches are undesirable, as they will introduce side-lobes when the data are processed (for example, when the data are correlated with the source signature or during seismic migration). A number of solutions exist to remove these side-lobes. Where signal to noise levels are good, deconvolution of the distributed sequence can be attempted such that the signal in the notches can be recovered. In another approach, where the time distribution of the airgun array varies from location to location, a spatial reconstruction can be attempted, where the information from neighboring sources is used to reconstruct the information missing in the spectral notches. These methods impose limitations on the use of the distributed airgun.
For instance, it is unlikely that signal to noise levels will be high enough to satisfactorily deconvolve the distributed signature. In this case errors will be introduced into the deconvolved result (noise will be amplified), or if appropriately stabilized, this will introduce sidelobes (note that the extreme case of stabilizing the deconvolution is simply to cross-correlate the data).
Spatial reconstruction becomes difficult where the source sampling interval results in spatially aliased data. Typically, airgun sources will be fired every 25 m, allowing the seismic wavefield to be well sampled (spatially un-aliased) on the source side to a frequency of only 30 Hz (if a water velocity of 1500 m/s is assumed); thus, beyond this frequency spatial reconstruction of the notch frequencies becomes difficult. This is especially true for the reconstruction of missing frequencies at a given source location, as the gap between the two sources’ either side is twice the shot interval. To have different time distribution for the airgun array from location to location, the airguns need to be activated according to different sequence from shot to shot.